Light is the most potent stimulus to shift the phase of the endogenous circadian timing system. If properly timed, it is effective in the treatment of circadian phase misalignment as seen in circadian rhythm sleep disorders (e.g., delayed sleep phase syndrome, advanced sleep phase syndrome, non-24-hour sleep-wake cycle, jet lag, and shift work). In order to optimize the properties of light exposure to activate neurological systems such as resetting of the endogenous circadian pacemaker, multiple variables of light stimuli have extensively been investigated (timing, intensity, wavelength, duration, number, and pattern of exposure). Interestingly, however, the effect of prior photic history on the sensitivity of the circadian timing system to light stimuli has received little attention. Animal experimental research has demonstrated that the circadian resetting response to a light stimulus can be reduced by a preceding non-saturating stimulus and recent research has also shown some evidence of modulation of melatonin suppression to prior lighting conditions in humans, both in the field and under controlled laboratory lighting conditions. In our preliminary data, we have demonstrated that prior photic history affects the sensitivity of circadian timing system, in that it modulates the strength of melatonin suppression and also the phase shifting response to light exposure. PUBLIC HEALTH RELEVANCE: Defining the influence of prior photic history on the circadian timing system will enable the optimization of light therapy in the treatment of circadian phase misalignment. Potentiating the photic sensitivity of the circadian pacemaker would facilitate rapid achievement of larger phase-shifts in response to light stimuli. Moreover, it may allow the use of a lower intensity of a light stimulus and/or a shorter duration of a light stimulus to achieve a similar effect. The understanding of the effects of prior photic history could help explain the ability of the circadian system to adapt to changes in natural outside light intensities across the seasons and at different latitudes.